How do sailboats manage to sail against the wind? Physics of motion of a sailing yacht What is the movement of a sailing vessel against the wind called?
No less important than the resistance of the hull is the traction force developed by the sails. To more clearly imagine the work of sails, let's get acquainted with the basic concepts of sail theory.
We have already talked about the main forces acting on the sails of a yacht sailing with a tailwind (jibed course) and a headwind (behind wind course). We found out that the force acting on the sails can be decomposed into the force that causes the yacht to roll and drift downwind, the drift force and the traction force (see Fig. 2 and 3).
Now let's see how the total force of wind pressure on the sails is determined and what the thrust and drift forces depend on.
To imagine the operation of a sail on sharp courses, it is convenient to first consider a flat sail (Fig. 94), which experiences wind pressure at a certain angle of attack. In this case, vortices are formed behind the sail, pressure forces arise on the windward side, and rarefaction forces arise on the leeward side. Their resulting R is directed approximately perpendicular to the plane of the sail. To properly understand the operation of a sail, it is convenient to imagine it as the resultant of two component forces: X-directed parallel to the air flow (wind) and Y-directed perpendicular to it.
The force X directed parallel to the air flow is called the drag force; It is created, in addition to the sail, also by the hull, rigging, spars and crew of the yacht.
The force Y directed perpendicular to the air flow is called lift in aerodynamics. It is this that creates thrust in the direction of movement of the yacht on sharp courses.
If, with the same drag of the sail X (Fig. 95), the lift force increases, for example, to the value Y1, then, as shown in the figure, the resultant of the lift force and drag will change by R and, accordingly, the thrust force T will increase to T1.
Such a construction makes it easy to verify that with an increase in drag X (at the same lift force), the thrust T decreases.
Thus, there are two ways to increase the traction force, and therefore the speed on sharp courses: increasing the lifting force of the sail and reducing the drag of the sail and the yacht.
In modern sailing, the lifting force of the sail is increased by giving it a concave shape with some “belliness” (Fig. 96): the size from the mast to the most deep place The "belly" is usually 0.3-0.4 of the sail's width, and the depth of the "belly" is about 6-10% of the width. The lifting force of such a sail is 20-25% greater than that of a completely flat sail with almost the same drag. True, a yacht with flat sails sails a little steeper into the wind. However, with potbellied sails, the speed of progress into the tack is greater due to the greater thrust.
Rice. 96. Sail profile
Note that with potbellied sails, not only the thrust increases, but also the drift force, which means that the roll and drift of yachts with potbellied sails is greater than with relatively flat ones. Therefore, a sail “bulge” of more than 6-7% in strong winds is unprofitable, since an increase in heel and drift leads to a significant increase in hull resistance and a decrease in the efficiency of the sails, which “eat up” the effect of increasing thrust. In weak winds, sails with a “belly” of 9-10% pull better, since due to the low total wind pressure on the sail, the heel is small.
Any sail at angles of attack greater than 15-20°, that is, when the yacht is heading 40-50° to the wind or more, can reduce lift and increase drag, since significant turbulence is formed on the leeward side. And since the main part of the lifting force is created by a smooth, turbulent-free flow around the leeward side of the sail, the destruction of these vortices should have a great effect.
The turbulence that forms behind the mainsail is destroyed by setting the jib (Fig. 97). The air flow entering the gap between the mainsail and the jib increases its speed (the so-called nozzle effect) and, when the jib is adjusted correctly, “licks” the vortices from the mainsail.
Rice. 97. Jib work
The profile of a soft sail is difficult to maintain constant at different angles of attack. Previously, dinghies had through battens running through the entire sail - they were made thinner within the “belly” and thicker towards the luff, where the sail is much flatter. Nowadays, through battens are installed mainly on ice boats and catamarans, where it is especially important to maintain the profile and rigidity of the sail at low angles of attack, when a regular sail is already lashing along the luff.
If the source of lift is only the sail, then drag is created by everything that ends up in the air flow flowing around the yacht. Therefore, improving the traction properties of the sail can also be achieved by reducing the drag of the yacht's hull, mast, rigging and crew. For this purpose, various types of fairings are used on the spar and rigging.
The amount of drag on a sail depends on its shape. According to the laws of aerodynamics, the drag of an aircraft wing is lower, the narrower and longer it is for the same area. That is why they try to make the sail (essentially the same wing, but placed vertically) high and narrow. This also allows you to use the upper wind.
The drag of a sail depends to a very large extent on the condition of its leading edge. The luffs of all sails should be covered tightly to prevent the possibility of vibration.
It is necessary to mention one more very important circumstance - the so-called centering of the sails.
It is known from mechanics that any force is determined by its magnitude, direction and point of application. So far we have only talked about the magnitude and direction of the forces applied to the sail. As we will see later, knowledge of the application points is of great importance for understanding the operation of sails.
Wind pressure is distributed unevenly over the surface of the sail (its front part experiences more pressure), however, to simplify comparative calculations, it is assumed that it is distributed evenly. For approximate calculations, the resultant force of wind pressure on the sails is assumed to be applied to one point; the center of gravity of the surface of the sails is taken as it when they are placed in the center plane of the yacht. This point is called the center of sail (CS).
Let's focus on the simplest graphical method for determining the position of the CPU (Fig. 98). Draw the sail area of the yacht on the required scale. Then, at the intersection of medians - lines connecting the vertices of the triangle with the midpoints of opposite sides - the center of each sail is found. Having thus obtained in the drawing the centers O and O1 of the two triangles that make up the mainsail and the staysail, draw two parallel lines OA and O1B through these centers and lay on them in opposite directions in any but the same scale as many linear units as square meters in the triangle; From the center of the mainsail the area of the jib is laid off, and from the center of the jib - the area of the mainsail. End points A and B are connected by straight line AB. Another straight line - O1O connects the centers of the triangles. At the intersection of straight lines A B and O1O there will be a common center.
Rice. 98. Graphical method of finding the center of sail
As we have already said, the drift force (we will consider it applied in the center of the sail) is counteracted by the lateral resistance force of the yacht’s hull. The lateral resistance force is considered to be applied at the center of lateral resistance (CLR). The center of lateral resistance is the center of gravity of the projection of the underwater part of the yacht onto the center plane.
The center of lateral resistance can be found by cutting out the outline of the underwater part of the yacht from thick paper and placing this model on a knife blade. When the model is balanced, lightly press it, then rotate it 90° and balance it again. The intersection of these lines gives us the center of lateral resistance.
When the yacht sails without heeling, the CP should lie on the same vertical straight line with the CB (Fig. 99). If the CP lies in front of the central station (Fig. 99, b), then the drift force, shifted forward relative to the force of lateral resistance, turns the bow of the vessel into the wind - the yacht falls away. If the CPU is behind the central station, the yacht will turn its bow to the wind, or be driven (Fig. 99, c).
Rice. 99. Yacht alignment
Both excessive adjustment to the wind, and especially stalling (improper centering) are harmful to the sailing of the yacht, as they force the helmsman to constantly work the helm to maintain straightness, and this increases hull drag and reduces the speed of the vessel. In addition, incorrect alignment leads to deterioration in controllability, and in some cases, to its complete loss.
If we center the yacht as shown in Fig. 99, and, that is, the CPU and the central control system will be on the same vertical, then the ship will be driven very strongly and it will become very difficult to control it. What's the matter? There are two main reasons here. Firstly, the true location of the CPU and central nervous system does not coincide with the theoretical one (both centers are shifted forward, but not equally).
Secondly, and this is the main thing, when heeling, the traction force of the sails and the longitudinal resistance force of the hull turn out to lie in different vertical planes (Fig. 100), it turns out like a lever that forces the yacht to be driven. The greater the roll, the more prone the vessel is to pitch.
To eliminate such adduction, the CP is placed in front of the central nervous system. The moment of traction and longitudinal resistance that arises with the roll, forcing the yacht to be driven, is compensated by the trapping moment of the drift forces and lateral resistance when the CP is located at the front. For good centering, it is necessary to place the CP in front of the CB at a distance equal to 10-18% of the length of the yacht along the waterline. The less stable the yacht is and the higher the CPU is raised above the central station, the more it needs to be moved to the bow.
In order for the yacht to have a good move, it must be centered, that is, put the CP and CB in a position in which the vessel on a close-hauled course in a light wind was completely balanced by the sails, in other words, it was stable on the course with the rudder thrown or fixed in the DP (allowed slight tendency to float in very light winds), and in stronger winds had a tendency to float. Every helmsman must be able to center the yacht correctly. On most yachts, the tendency to roll increases if the rear sails are overhauled and the front sails are loose. If the front sails are overhauled and the rear sails are damaged, the ship will sink. With an increase in the “belliness” of the mainsail, as well as poorly positioned sails, the yacht tends to be driven to a greater extent.
Rice. 100. The influence of heel on bringing the yacht into the wind
The winds that are in the southern part Pacific Ocean blowing in a westerly direction. That is why our route was designed so that on the sailing yacht “Juliet” we move from east to west, that is, so that the wind blows at our back.
However, if you look at our route, you will notice that often, for example when moving from south to north from Samoa to Tokelau, we had to move perpendicular to the wind. And sometimes the direction of the wind changed completely and we had to go against the wind.
Juliet's route
What to do in this case?
Sailing ships have long been able to sail against the wind. The classic Yakov Perelman wrote about this long ago well and simply in his second book from the series “Entertaining Physics”. I present this piece here verbatim with pictures.
"Sailing against the wind
It is difficult to imagine how sailing ships can go “against the wind” - or, as sailors say, go “close-hauled”. True, a sailor will tell you that you cannot sail directly against the wind, but you can only move at an acute angle to the direction of the wind. But this angle is small - about a quarter of a right angle - and it seems, perhaps, equally incomprehensible: whether to sail directly against the wind or at an angle to it of 22°.
In reality, however, this is not indifferent, and we will now explain how it is possible to move towards it at a slight angle by the force of the wind. First, let's look at how the wind generally acts on the sail, that is, where it pushes the sail when it blows on it. You probably think that the wind always pushes the sail in the direction it blows. But this is not so: wherever the wind blows, it pushes the sail perpendicular to the plane of the sail. Indeed: let the wind blow in the direction indicated by the arrows in the figure below; the line AB represents the sail.
The wind always pushes the sail at right angles to its plane.
Since the wind presses evenly on the entire surface of the sail, we replace the wind pressure with a force R applied to the middle of the sail. We will split this force into two: force Q, perpendicular to the sail, and force P, directed along it (see figure above, right). The last force pushes the sail nowhere, since the friction of the wind on the canvas is negligible. The force Q remains, which pushes the sail at right angles to it.
Knowing this, we can easily understand how a sailing ship can sail at an acute angle towards the wind. Let line KK represent the keel line of the ship.
How can you sail against the wind?
The wind blows at an acute angle to this line in the direction indicated by a series of arrows. Line AB represents a sail; it is placed so that its plane bisects the angle between the direction of the keel and the direction of the wind. Follow the distribution of forces in the figure. We represent the wind pressure on the sail by force Q, which, we know, must be perpendicular to the sail. Let us divide this force into two: force R, perpendicular to the keel, and force S, directed forward along the keel line of the ship. Since the movement of the vessel in the direction R encounters strong resistance from the water (keel in sailing ships becomes very deep), then the force R is almost completely balanced by the resistance of the water. There remains only one force S, which, as you see, is directed forward and, therefore, moves the ship at an angle, as if towards the wind. [It can be proven that the force S is greatest when the plane of the sail bisects the angle between the keel and wind directions.]. Typically this movement is performed in zigzags, as shown in the figure below. In the language of sailors, such a movement of the ship is called “tacking” in the strict sense of the word."
Let's now consider all possible wind directions relative to the boat's heading.
Diagram of the ship's course relative to the wind, that is, the angle between the wind direction and the vector from stern to bow (course). 
When the wind blows in your face (leventik), the sails dangle from side to side and it is impossible to move with the sail. Of course, you can always lower the sails and turn on the engine, but this no longer has anything to do with sailing.
When the wind blows directly behind you (jibe, tailwind), the accelerated air molecules put pressure on the sail on one side and the boat moves. In this case, the ship can only move slower than the wind speed. The analogy of riding a bicycle in the wind works here - the wind blows at your back and it is easier to turn the pedals.
When moving against the wind (close-hauled), the sail moves not because of the pressure of air molecules on the sail from behind, as in the case of a jibe, but because of the lifting force that is created due to different air velocities on both sides along the sail. Moreover, because of the keel, the boat does not move in a direction perpendicular to the course of the boat, but only forward. That is, the sail in this case is not an umbrella, as in the case of a close-hauled sail, but an airplane wing.
During our passages, we mainly sailed with backstays and gulfwinds at an average speed of 7-8 knots with a wind speed of 15 knots. Sometimes we sailed against the wind, halfwind and close-hauled. And when the wind died down, they turned on the engine.
In general, a boat with a sail going against the wind is not a miracle, but a reality.
The most interesting thing is that boats can sail not only against the wind, but even faster than the wind. This happens when the boat backstays, creating its own wind.
We continue the series of publications prepared by the interactive popular science blog “I’ll Explain in Two Minutes.” The blog talks about simple and complex things that surround us every day and do not raise any questions until we think about them. For example, there you can find out how spaceships do not miss or collide with the ISS during docking.
1. It is impossible to sail strictly against the wind. However, if the wind is blowing from the front, but slightly at an angle, the yacht may well move. In such cases, the ship is said to be sailing on a sharp course.
2. The thrust of a sail is generated by two factors. Firstly, the wind simply presses on the sails. Secondly, the oblique sails installed on most modern yachts, when air flows around them, work like an airplane wing and create “lifting force”, only it is directed not upward, but forward. Due to aerodynamics, the air on the convex side of the sail moves faster than on the concave side, and the pressure on the outside of the sail is less than on the inside.
3. The total force created by the sail is directed perpendicular to the canvas. According to the rule of vector addition, it is possible to distinguish the drift force (red arrow) and the traction force (green arrow).
4. On sharp courses, the drift force is great, but it is countered by the shape of the hull, keel and rudder: the yacht cannot go sideways due to water resistance. But it willingly slides forward even with a small traction force.
5. To sail strictly against the wind, the yacht tacks: it turns to the wind first on one side or the other, moving forward in segments - tacks. How long the tacks should be and at what angle to the wind to go are important issues of skipper tactics.
6. There are five main courses of a ship relative to the wind. Thanks to Peter I, Dutch maritime terminology took root in Russia.
7. Leventik－ the wind blows directly at the bow of the ship. It is impossible to sail this way, but turning to the wind is used to stop the yacht.
8. Closed wind－ the same acute course. When you go close-hauled, the wind blows in your face, so it seems that the yacht is developing a very high speed. In fact, this feeling is deceptive.
9. Gulfwind－ the wind blows perpendicular to the direction of movement.
10. Backstay－ the wind blows from the stern and from the side. This is the fastest course. Fast racing boats sailing backstayed can accelerate to speeds exceeding the speed of the wind due to the lifting force of the sail.
11. Fordewind－ the same tailwind blowing from the stern. Contrary to expectations, it is not the fastest course: here the lifting power of the sail is not used, and the theoretical speed limit does not exceed the speed of the wind. An experienced skipper can predict invisible air currents just like an airplane pilot can predict updrafts and downdrafts.
You can view an interactive version of the diagram on the “I’ll Explain in Two Minutes” blog.
Movement sailing yacht downwind is actually determined by the simple pressure of the wind on her sail, pushing the ship forward. However, wind tunnel research has shown that sailing upwind exposes the sail to a more complex set of forces.
When the incoming air flows around the concave rear surface of the sail, the air speed decreases, while when flowing around the convex front surface of the sail, this speed increases. As a result, an area of high pressure is formed on the back surface of the sail, and a low pressure area on the front surface. The pressure difference on the two sides of the sail creates a pulling (pushing) force that moves the yacht forward at an angle to the wind.
A sailing yacht located approximately at right angles to the wind (in nautical terminology, the yacht is tacked) moves quickly forward. The sail is subject to pulling and lateral forces. If a sailing yacht sails at an acute angle to the wind, its speed slows down due to a decrease in the pulling force and an increase in the side force. The more the sail is turned towards the stern, the slower the yacht moves forward, in particular due to the large lateral force.
A sailing yacht cannot sail directly into the wind, but it can move forward by making a series of short zigzag movements at an angle to the wind, called tacks. If the wind blows to the left side (1), the yacht is said to be sailing on port tack; if it is blowing to starboard (2), it is said to be sailing on starboard tack. In order to cover the distance faster, the yachtsman tries to increase the speed of the yacht to the limit by adjusting the position of its sail, as shown in the figure below left. To minimize deviation to the side from a straight line, the yacht moves, changing course from starboard tack to port and vice versa. When the yacht changes course, the sail is thrown to the other side, and when its plane coincides with the wind line, it flutters for some time, i.e. is inactive (middle picture below the text). The yacht finds itself in the so-called dead zone, losing speed until the wind again inflates the sail from the opposite direction.
Before looking at how a sail works, there are two short but important points to consider:
1. Determine what kind of wind affects the sails.
2.Talk about specific marine terminology associated with courses relative to the wind.
True and apparent winds in yachting.
The wind that acts on a moving ship and everything on it is different from the one that acts on any stationary object.
Actually, the wind as an atmospheric phenomenon blowing relative to land or water is what we call true wind.
In yachting, the wind relative to a moving yacht is called apparent wind and is the sum of the true wind and the oncoming air flow caused by the movement of the vessel.
The apparent wind always blows at a sharper angle to the boat than the true wind.
The apparent wind speed can be greater (if the true wind is headwind or sidewind), or less than the true wind (if it is from a tailwind).
Directions relative to the wind.
In the wind means from the direction from which the wind blows.
Downwind- from the direction the wind blows.
These terms, as well as derivatives from them, such as “windward”, “leeward”, are used very widely, and not only in yachting.
When these terms are applied to a ship, it is customary to also talk about the windward and leeward sides.
If the wind blows from the starboard side of the yacht, then this side is called windward, left side - leeward respectively.
Port and starboard tack are two terms directly related to the previous ones: if the wind blows to the starboard side of the ship, then they say that it is sailing on the starboard tack, if it is on the left, then on the left.
In English nautical terminology, what is associated with starboard and port is different from the usual Right and Left. They say Starboard about the starboard side and everything related to it, and Port about the left side.
Courses relative to the wind.
Courses relative to the wind vary depending on the angle between the direction of the apparent wind and the direction the vessel is moving. They can be divided into acute and full.
Close-hauled is a sharp course relative to the wind. when the wind blows at an angle of less than 80°. There can be a steep close-hauled wind (up to 50°) or a full close-hauled wind (from 50 to 80°).
Full courses relative to the wind are courses when the wind blows at an angle of 90° or more to the direction the yacht is moving.
These courses include:
Gulfwind - the wind blows at an angle of 80 to 100°.
Backstay - the wind blows at an angle from 100 to 150° (steep backstay) and from 150 to 170° (full backstay).
Fordewind - the wind blows astern at an angle of more than 170°.
Leftist - the wind is strictly headwind or close to it. Since a sailing ship cannot move against such a wind, it is more often called not a course, but a position relative to the wind.
Maneuvers relative to the wind.
When a yacht under sail changes its course so that the angle between the wind and the direction of motion decreases, then the ship is said to be is given. In other words, to flatten means to go at a sharper angle to the wind.
If the reverse process occurs, i.e. the yacht changes course towards increasing the angle between it and the wind, the ship falls away .
Let us clarify that the terms (“lead” and “fall”) are used when the boat changes course relative to the wind within the same tack.
If the ship changes tack, then (and only then!) such a maneuver in yachting is called a turn.
There are two different ways to change tack and, accordingly, two turns: tack And jibe .
A tack is a turn into the wind. The vessel is driven, the bow of the boat crosses the wind line, at some point the vessel passes through the left-hand position, after which it lies on the other tack.
Yachting when jibes occurs in the opposite way: the ship falls away, the stern crosses the wind line, the sails are transferred to the other side, the yacht lies on a different tack. Most often this is a turn from one full course to another.
Sail operation during yachting.
One of the main challenges for a sailor when working with sails is to orient the sail at the optimal angle relative to the wind to best propel the sail forward. To do this, you need to understand how the sail interacts with the wind.
The work of a sail is in many ways similar to the work of an airplane wing and occurs according to the laws of aerodynamics. For particularly curious yachtsmen, you can learn more about the aerodynamics of a sail as a wing in a series of articles:. But it’s better to do this after reading this article, gradually moving from easy to more complex material. Although, who am I telling this to? Real yachtsmen are not afraid of difficulties. And you can do everything exactly the opposite.
The main difference between a sail and an aircraft wing is that for an aerodynamic force to appear on the sail, a certain non-zero angle is needed between it and the wind; this angle is called the angle of attack. The airplane wing has an asymmetrical profile and can operate normally at zero angle of attack, but the sail does not.
As the wind flows around the sail, an aerodynamic force arises, which ultimately moves the yacht forward.
Let's consider the operation of a sail in yachting at different courses relative to the wind. First, for simplicity, let's imagine that a mast with one sail is dug into the ground and we can direct the wind at different angles to the sail.
Angle of attack 0°. The wind blows along the sail, the sail flutters like a flag. There is no aerodynamic force on the sail, there is only drag force.
Angle of attack 7°. An aerodynamic force begins to appear. It is directed perpendicular to the sail and is still small in size.
The angle of attack is about 20°. The aerodynamic force has reached its maximum value and is directed perpendicular to the sail.
Angle of attack 90°. Compared to the previous case, the aerodynamic force did not change significantly either in magnitude or direction.
Thus, we see that the aerodynamic force is always directed perpendicular to the sail and its magnitude practically does not change in the angle range from 20 to 90°.
Angles of attack greater than 90° do not make sense to consider, since the sails on a yacht are usually not set at such angles relative to the wind.
The above dependences of the aerodynamic force on the angle of attack are largely simplified and averaged.
In fact, these properties vary markedly depending on the shape of the sail. For example, a long, narrow and fairly flat mainsail of racing yachts will have a maximum aerodynamic force at an angle of attack of about 15°; at higher angles the force will be slightly less. If the sail is more potbellied and does not have a very large aspect ratio, then the aerodynamic force on it can be maximum at an angle of attack of about 25-30°.
Now let's look at how a sail works on a yacht.
For simplicity, let's imagine that there is only one sail on the yacht. Let it be a grotto.
First, it’s worth looking at how the yacht + sail system behaves when moving on the sharpest courses relative to the wind, since this usually raises the most questions.
Let’s say the yacht is affected by wind at an angle of 30-35° to the hull. By orienting the sail on course at an angle of approximately 20° to the wind, we obtain a sufficient aerodynamic force A on it.
Since this force acts at right angles to the sail, we see that it pulls the yacht strongly to the side. By decomposing the force A into two components, you can see that the forward thrust force T is several times less than the force pushing the boat sideways (D, drift force).
What causes the yacht to move forward in this case?
The fact is that the design of the underwater part of the hull is such that the resistance of the hull to movement to the side (the so-called lateral resistance) is also several times greater than the resistance to movement forward. This is facilitated by the keel (or centreboard), rudder and the very shape of the hull.
However, lateral resistance occurs when there is something to resist, i.e., for it to start working, some sideways displacement of the body, the so-called wind drift, is required.
This displacement naturally occurs under the action of the lateral component of the aerodynamic force, and as a response, a lateral drag force S immediately arises, directed in the opposite direction. As a rule, they balance each other at a drift angle of about 10-15°.
So, it is obvious that the lateral component of the aerodynamic force, most pronounced on sharp courses relative to the wind, causes two undesirable phenomena: wind drift and roll.
Wind drift means that the yacht's trajectory does not coincide with its centreline (diameter plane, or DP, is a smart term for the bow-stern line). There is a constant shift of the yacht to the wind, moving as if a little sideways.
It is known that when yachting on a close-hauled course at average weather conditions wind drift as an angle between the DP and the actual trajectory of movement is approximately 10-15°.
Advance against the wind. Tacking.
Since yachting under sails is not possible strictly against the wind, and you can only move at a certain angle, it would be good to have an idea of how sharply the yacht can move in degrees to the wind. And what, accordingly, is that slow sector of courses relative to the wind, in which movement against the wind is impossible.
Experience shows that a regular cruising yacht (not a racing yacht) can effectively sail at an angle of 50-55° to the true wind.
Thus, if the goal that needs to be achieved is located strictly against the wind, then yachting to it will not take place in a straight line, but in a zigzag, first on one tack, then on the other. In this case, on each tack, naturally, you will need to try to sail as sharply as possible into the wind. This process is called tacking.
The angle between the trajectories of yachts on two adjacent tacks when tacking is called tacking. Obviously, with a sharpness of movement to the wind of 50-55°, the tacking angle will be 100-110°.
The magnitude of the tacking angle shows us how effectively we can move towards the target if it is strictly against the wind. For an angle of 110°, for example, the path to the target increases by 1.75 times compared to moving in a straight line.
Sail operation on other courses relative to the wind
It is obvious that already on a gulfwind course the thrust force T significantly exceeds the drift force D, so the drift and roll will be small.
With the backstay, as we see, not much has changed compared to the gulfwind course. The mainsail is placed in a position almost perpendicular to the DP, and this position is extreme for most yachts; it is technically impossible to deploy it even further.
The position of the mainsail on the gybe course is no different from the position on the backstay course.
Here, for simplicity, when considering the physics of the process in yachting, we take into account only one sail - the mainsail. Typically, a yacht has two sails - a mainsail and a staysail (headsail). So, on a gybe course, the jib (if it is located on the same side as the mainsail) is in the wind shadow of the mainsail and practically does not work. This is one of several reasons why jibes are not a favorite among boaters.
